Corrosion-resistant nickel-chromium alloys with improved stress-rupture characteristics



United States Patent ABSTRACT OF THE DISCLOSURE A corrosion-resistant nickel-chromium-base alloy of high chromium content (27% to 35%) possessing good stress-rupture properties at elevated temperatures and containing controlled amounts of titanium, aluminum, cobalt, molybdenum, carbon, boron and zirconium.

The present invention relates to nickel-chromium base alloys suitable for use under stress at elevated temperatures, and, more particularly, to high chromium, nickel alloys which manifest a combination of desired high temperature properties, including enhanced resistance to corrosive environments, such as the combustion products of impure fuel, and a satisfactory level of stress-rupture characteristics.

As is well known to those skilled in the art, nickelchromium base alloys have found wide and diverse utilization in industry, particularly for high temperature applications wherein substantial resistance to stress-rupture is required. Such alloys as commercially employed typically contain the precipitation hardening elements titanium and aluminum, and chromium in an amount of about to although a review of recent literature seemingly indicates a trend to using lesser amounts of chromium in certain nickel alloys to achieve a higher magnitude of stress-rupture characteristics. This accomplishment has usually been at the expense of resistance to corrosion.

In our British Patent No. 959,509, nickel alloys containing substantial amounts of chromium, to wit, 27% to are disclosed as having excellent resistance to corrosive environments. These alloys contained, in addition to chromium, about 1.5% to 2.5% titanium, about 0.5% to 1.1% aluminum, the combined amount of titanium plus aluminum being from 2% to 3.2%. It is also disclosed therein that while it might have been thought that higher amounts of titanium and aluminum would have resulted in a higher order of stressupture properties, the reverse was the case. In any event, these alloys manifested a stress-rupture life of the order of to 140 hours when tested under a stress of 17 long tons per square inch (t.p.s.i.) at 750 C. in the wrought condition upon age hardening.

Now, notwithstanding the excellent resistance to corrosion afforded by the alloys described in the abovementioned British patent, for some purposes an improved level of stress-rupture properties is also required. It has now been discovered that marked improvement in the stress-rupture properties can be achieved without loss of corrosion resistance in such alloys through the incorporation of special amounts of both cobalt and molybdenum therein.

It is an object of the present invention to provide high chromium, nickel-base alloys which manifest excellent resistance to corrosive environments and which also exhibit a satisfactory level of high temperature stress-rupture properties.

Other objects and advantages will become apparent from the following description.

Generally speaking, the nickel-chromium base alloys of the present invention contain (by weight) about 27% to about 35% chromium, about 1.5% to about 2.5% titanium, about 0.5% to about 1.1% and preferably from 0.5% to 0.85% aluminum, the total titanium plus aluminum being from 2% to 3.2% and preferably from 2.2% to about 2.75%, from about 0.01% to 0.1% carbon, from about 0.001% to 0.01% boron, from about 0.01% to 0.1% zirconium, from 12% to 30% cobalt, from 1% to 7% molybdenum and the balance essentially nickel. The use of the expression balance or balance essentially in referring to the nickel content of the alloys, as will be understood by those skilled in the .art, does not exclude the presence of other elements commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities normally associated therewith in small amounts which do not adversely aifect the novel characteristics of the alloys. In this regard, silicon should not exceed about 1% and manganese and iron should not exceed 1% and 2%, respectively, the level of impurities otherwise being preferably kept as low as possible.

In carrying the invention into practice, the alloys can be prepared by conventional processing techniques well known to those skilled in the art, although vacuum melting is preferred. With regard to the heat treatment of the alloys, generally speaking, it comprises solution heating the alloys for about one to eight hours at a temperature within the range of 1050 C. to 1200 C., the alloys, upon cooling, being thereafter aged by heating for one to 24 hours within the temperature range of 600 C. to 800 C. A suitable heat treatment consists in solution heating for four hours at 1160 C. followed by air cooling and aging for 16 hours at 750 C. and again air cooling.

For the purpose of giving those skilled in the art a better appreciation and/ or understanding of the invention, the following illustrative data are given:

A series of alloys were prepared in wrought form, the compositions being given in Table 1. Alloys Nos. 1 through 5 are outside the scope of the invention, Alloy No. 6 being within the invention. Each of the alloys was tested for stress-rupture characteristics by subjecting wrought specimens to a stress of 17 t.p.s.i. at 750 C., the alloys having been solution heated for four hours at 1160 C., air cooled, aged for sixteen hours at 700 C. and again air cooled. The life-to-rupture in hours and tensile elongation in per cent are also given in Table I:

TABLE I Composition (percent by weight) Stress, Rupture Properties at 17 t.s.i./ 750 C.

Alloy No.

C Cr Ti Al Ti-l-Al 00 Mo Sr Zr 13 Hours El.

(percent) Stress, Rupture Properties at 17 t.s.i./750 C.

B Hours El.

(percent) our British Patent No. 959,509, that the contents of titanium and aluminum, both individually and combined, be within the ranges specified herein, since departure therefrom, at least in any appreciable degree, leads to a marked fall in the stress-rupture properties. This is clearly shown by the results in Table III which relate to alloys heat treated and tested in the same manner as the alloys of Tables I and II. Alloys Nos. 6, 11, and 15 in this table are in accordance with the invention, the others are 10 not. I

TABLE III Composition 1 (percent by weight) Cr Ti Al Ti+Al 00 Mo Zr The data of Table I clearly illustrate the beneficial effect in stress-rupture properties achieved with the co- Alloy No.

presence of cobalt and molybdenum. Alloy No. 3 which contained about 19% cobalt but which was molybdenumfree did not afford any appreciable increase in stressrupture life over those alloys devoid of cobalt. The same is true with regard to the cobalt-free but molybdenum containing Alloys Nos. 4 and 5. However, the co-presence of molybdenum and cobalt resulted in the marked increase of stress-rupture life, the increase being well over 150% of the stress-rupture lives of Alloys Nos. 1 and 2.

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Alloys of the present invention are suitable for use about as rotor blades and other stressed parts of gas turbines.

The alloys are particularly useful in forming articles subjected in use to high stress and corrosive atmospheres at elevated temperatures, for example, parts of furnaces and air heaters. Apart from being used in the Wrought 5 form, the alloys can also be used for cast components.

0.002% or 0.003% and up to about 0.006% boron 0.02% to 0.08% zirconium, with the balance being essentially nickel. Lower amounts of carbon, i.e., not more than 0.06%, can be suitably used particularly with aluminum contents of 0.7% to 1.1%.

It is important, as in the case of the alloys described in 7 Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications .and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium-base alloy characterized by a high degree of corrosion resistance and stress-rupture characteristics at elevated temperatures, said alloy consisting essentially of about 27% to 35% chromium, about 1.5% to about 2.5% titanium, about 0.5% to about 1.1% aluminum, the total titanium plus aluminum being from about 2.2% to 2.75%, carbon at least 0.01% to about 0.1% with the proviso that the aluminum content is at least about 0.7% when the carbon content is in the range of 0.01% to 0.06%, about 14% to about 20% cobalt,

6 about 3% to 6% molybdenum, about 0.001% to 0.006% boron, about 0.02% to about 0.08% zirconium and the balance essentially nickel.

2. An alloy in accordance with claim 1 containing at least 0.7% aluminum, at least 0.06% carbon, .and at least 16% cobalt.

References Cited UNITED STATES PATENTS 3,177,075 4/1965 Richards et a1 75l71 3,207,599 9/1965 Franklin et al 75l7l 3,222,165 12/1965 Bird et a1 75171 FOREIGN PATENTS 959,509 6/ 1964 Great Britain.

RICHARD O. DEAN, Primary Examiner.

mg? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 108, 1Y9 Dated August '2', 1969 Inventor) HARRY LEWIS and RONALD ALFRED SMITH It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 8, for "15" first occurrence, read -l l--.

SIGNED AND SEALED DEC 2 1969 (SEAL) Attest:

Edward M. Fletcher, Ir. Atteating Officer R. m.

Oomissianar of Patents 

